It has been common practice for many years to run a continuous reeled pipe (known extensively in the industry as "coil tubing") into a well to perform operations utilising the circulation of treating and cleanout fluids such as water, oil, acid, corrosion inhibitors, hot oil, nitrogen, foam, etc. Coil tubing, being continuous rather than jointed, is run into and out of a well with continuous movement of the tubing through a coil tubing injector.
Coil tubing is frequently used to circulate cleanout fluids through a well for the purpose of eliminating sand bridges, scale, and similar downhole obstructions. Often such obstructions are very difficult and occasionally impossible to remove because of the inability to rotate the coil tubing and drill out such obstructions. These well tubulars vary from unperforated and perforated pipe, large diameter casing, production tubing, and slotted or wire-wrapped well liner. Well tubulars often become plugged or coated with corrosion products, sediments and hydrocarbon deposits. The deposits may consist of silicates, sulphates, sulphide, carbonates, calcium, and organic growth.
It is desirable to perform drilling type operations in wells through use of coil tubing which can be run into and removed from a well quickly in addition to performing the usual operations which require only the circulation of fluids. The same types of well servicing can also be performed with various small diameter work strings. The present invention may be used with such work strings and is not limited to coil tubing.
High pressure fluid jet systems have been used for many years to clean the inside diameter of well tubulars. Examples of such systems are disclosed in the following U.S. Pat. Nos.: 3,720,264, 3,811,499, 3,829,134, 3,850,241, 4,088,191, 4,349,073, 4,441,557, 4,442,899, 4,518,041, 4,919,204, 5,181,576 or 5,337,819.
The abrasive of choice in current practice of well cleaning is sand, though other abrasive particless are known from different technical fields. For example, the use of non-spherical flint or steel shot is disclosed in the U.S. Pat. No. 4,482,392. The hardness of the material described is well above 50 on a Rockwell C scale.
A well cleaning method using coiled tubing is described in the International Patent Application WO 91/11270. It comprises the use of an abrasive mixture of carrier fluid and abrasive particles, a pumping system to pressurise said mixture and coiled tubing unit with a jetting head. The abrasive is characterised as rounded and its effect on the pipes is described as being confined to a beneficial shot-peening action. No specific example of such an abrasive is given.
In view of the above cited prior art it is an object of the invention is to provide a improved abrasive for well cleaning applications.